This invention relates to a pattern forming method.
Recently, with the promotion of higher density integration of semiconductor devices, the light source wavelength of an exposure device used for fine process, especially photolithography is more and more shortened. At present i-line (365 nm) has entired its practical use and KrF excimer laser (248.4 nm) is already investigated. However, resist pattern materials, especially resist materials suitable for KrF excimer laser, and deep UV wavelength ranges have not been developed sufficiently. For example, even when MP2400 (Shipley Co.) which is said to have high sensitivity and transmittance for KrF excimer laser light is used, because of the surface absorption of a novolac resin which is a base polymer and poor optical reactivity of a sensitizer, a naphthoquinone diazide compound, pattern profile after pattern formation is too low in quality to be used.
Also as a pattern forming material for deep UV, there is reported a resist which contains of 2-diazo-1,3-dione compound with high transmittance for deep ultraviolet light of near 248.4 nm. However, compared with the transmittance of 70% of the base polymer of the resist, the transmittance of the pattern forming material after exposure is only 45%, and enough photobleach is not obtained. Also as a result of pattern forming experiments, it is found that the pattern has an angle of approx. 70 degrees which value is insufficient compared with a pattern shape which becomes a satisfying etching mask with a vertical shape.
Also it has become clear that the sensitivity of this pattern forming material is as low as from 140 to 150 mJ/cm.sup.2. That is, the high transparent pattern forming material containing a 2-diazo-1,3-dione compound has low sensitivity, and is difficult for a practical use when there is used KrF excimer laser light of which energy efficiency is poor.
In recent years, as a means to decrease exposure energy quantity, a material comprising poly(tert-butoxy carbonate (t-BOC)) styrene and an onium salt was proposed. This is a chemical amplification pattern forming material which generates an acid by exposure to light, said acid acting as a catalyst. Various reports have been made recently [e.g. Polym. Eng. Sci. volume 23, page 1012 (1983)]. A pattern forming method using such a known chemical amplification pattern forming material is explained referring to FIG. 1. The pattern forming material 12 is spin-coated on a semiconductor substrate 1, and it is soft baked for 90 seconds on a hot plate of 90.degree. C. to obtain a film of pattern forming material of 1.0 .mu.m thick (FIG. 1(a)). In most cases, an insulation film, an electroconductive film and an oxide film are formed on the substrate 1. Next, an acid is generated from a photoacid generator in the material 12 as shown in the following chemical change by exposure to KrF excimer laser (248.4 nm) 4 through a mask 5 (FIG. 1(b)). ##STR1## Then, the chemical change mentioned below is caused by the heat treatment (post exposure bake, i.e. PEB) of the said material film on a hot plate 3 for 90 seconds at 130.degree. C., and the resin becomes alkali soluble (FIG. 1(c)). ##STR2## Finally, positive type pattern 12a and 12c are obtained by dissolution and removal of exposed part 12 of the pattern forming material 12 by using an alkaline developer (MF-319, mfd. by Shipley Co.) (FIG. 1(d)).
But it was found impossible to apply this method to formation of fine patterns 12c on the substrate as in FIG. 1(d), if the pattern size is 1.0 .mu.m or less, especially 0.5 .mu.m or less. That is, a fine resist pattern 12, which is to be retained, is not retained. A reason why super fine patterns are not formed was proved to be the low adhesiveness between the pattern forming material and the substrate according to the present inventors' investigation. Although this is not a problem in a device production of several .mu.m level as in the said instance, this is a fatal problem in a process for forming fine patterns of 1 .mu.m or less, especially super fine patterns of 0.5 .mu.m or less in high density. Consequently, it is impossible to produce a device of submicron rule. Thus, the reason why the super fine patterns are not formed turned out to be the low adhesiveness between the pattern forming material and the substrate. Since poly (t-BOC)styrene resin conventionally used for a pattern forming material does not contain a hydrophilic radical in its molecule, when it is made into a thin film, it becomes hydrophobic. As for the substrate, since hydrophobic treatment with hexamethyldisilazane (HMDS) is performed before forming a pattern forming material film, the substrate surface is hydrophobic. Since the hydrophobic substrate and the hydrophobic pattern forming material have poor adhesiveness, the exposed part of the resist material is dissolved and removed during development after patternwise exposure. At the same time, an unexposed part of the resist material, which should not be dissolved, is removed because of poor adhesiveness, and does not remain on the substrate. This phenomenon becomes especially remarkable in fine patterns of 1 .mu.m or less. Therefore, it is extremely important to prevent this in the production of highly profitable super fine semiconductor integrated circuits having super fine patterns of 1 .mu.m or less, particularly 0.5 .mu.m or less.